Our invention relates to a method and apparatus for fine tuning a coupled-dual resonator crystal and, more particularly, for selectively controlling the two resonator frequencies and the coupling between the resonators of a coupled-resonator crystal.
Crystal filters are used extensively in radio and electronic circuits. Dual or two-resonator crystal filters are preferred in many applications because they provide the characteristics of a very narrow bandpass filter or part of a very narrow filter, due to the extremely high Q of the associated resonator crystals, in a relatively small size. Such very narrow filter characteristics are virtually impossible or extremely difficult to realize with lumped devices, such as inductors or capacitors.
A typical dual resonator crystal filter includes a wafer of piezoelectric material having two or more pairs of electrodes formed on the wafer. The mass loading of each pair of electrode (one on each side forming a resonator) determines in part the resonant frequency of that resonator. Coupling between the pair of electrodes forming the input resonator and the pair of electrodes forming the output resonator is determined in part by the effective separation between the resonators. A coupling strip may be positioned on one face of the piezoelectric material between the input and output electrodes to reduce the effective separation and thus increase coupling.
The three major characteristics of the coupled-dual crystal which are controllable during its fabrication process and which are primary determinants of the filtering characteristics of the corresponding filter are the resonator frequencies of the input and output resonators and the synchronous peak separation frequency (SPSF). Synchronous peak separation frequency is a measure of coupling and has been defined in U.S. Pat. No. 4,093,914, which is assigned to the same assignee as the present invention and incorporated herein by reference, as the absolute value of the difference between the two short circuit resonant frequencies that would occur if the two resonators were tuned to the same resonant frequency.
As taught by U.S. Pat. No. 4,093,914, the significance of synchronous peak separation frequency is that it can provide a common reference value at a particular point in the process of tuning a specific crystal for evaluating the acoustic coupling of that crystal. It has meaning in that it represents a value or number which can be related mathematically to the coupling capacitor in the equivalent circuit of the crystal. It also provides a convenient measure of a specific crystal's coupling capability, as related to the nominal design, independent of the actual variations in resonator frequencies of the crystal. The value of the synchronous peak separation frequency is that it provides a means for controlling or monitoring the fabrication of crystals of a specific coupled-dual crystal design by mathematically relating the acoustic coupling of each crystal to standard or common conditions. It represents, in essence, a "coupling parameter" which can be used and calculated at any point in the fabrication process regardless of whether or not the two resonators are equal in frequency. It can be distinguished from the filter resonator coupling of Thompson (U.S. Pat. No. 4,343,827) which is associated with balanced open circuit resonant frequencies. By using synchronous peak separation frequency instead of Thompson's filter resonator coupling, we have a coupling parameter which we can use anywhere in the tuning process regardless of whether or not the resonator frequencies are equal. Thus, we use synchronous peak separation frequency in our process and, as used hereinafter, the terms "resonator coupling", "coupling parameter" and "coupling" refer to synchronous peak separation frequency (or SPSF).
The prior art teaches that coupling and the center or mean frequency (the arithmetic mean of the two resonator frequencies) can be adjusted or "tuned" by adding mass to the electrodes of the resonators to decrease their respective frequencies. However, because a Change in mass loading of the electrodes simultaneously varies both parameters, the final frequency adjustment of SPSF and mean frequency becomes difficult.
Several techniques have been developed in the past in an attempt to efficiently accomplish this fine-tuning of crystal filters. One such technique for fine-tuning monolithic crystal filters is disclosed in the aforementioned U.S. Pat. No. 4,343,827 in which additional mass is deposited on just one side of the crystal wafer. First, additional electrode material is plated on one half of a solid electrode (on the grounded side) to balance the open circuit resonator resonant frequencies of the filter. Additional electrode material is then plated on the entire solid electrode through a plate mask larger than the entire ground side electrode to fine-tune the midband frequency to its final desired value. The difficulty with this system is that the platings tend to overlap the conductor pattern to cause changes in inductance because no mask is actually indexed off of or fits over the crystal. Also implicit in this patent is a frequency monitoring technique which is slow and likely to be inaccurate in that it monitors amplitude via an oscilloscope to balance the frequencies of the resonators, which is a limitation in itself.
J. L. Hokanson has suggested (in "Laser Machining Thin Film Electrode Arrays on Quartz Crystal Substrates", 23rd Annual Symposium on Frequency Control, May 1969) a technique for adjusting coupling via a coupling stripe between electrodes using a laser for machining off material In addition, the resonator frequencies are adjusted using laser machining of the resonator electrodes. This same technique is proposed by Watanabe and Tsukamoto in a somewhat more refined manner (see "High-Performance Monolithic Crystal Filters with Stripe Electrodes", Electron Commun Japan, Vol. 57 part A, pp. 53-60). One problem with these techniques is the relatively high cost of the laser equipment necessary to implement them.
Another problem is that even with prior art frequency tuning systems, the method and apparatus for controlling the amount of plating on the various components have proven inadequate. Typically, the resonators are plated in a vacuum chamber in which a piece of metal, such as high purity silver, is vaporized and deposited on the resonator electrodes. Control of the location and duration of the plating is critical. In the typical system, the resonators are masked to expose only the portion of the resonator to be plated, thus fixing the location of the plating. As for the duration, in some systems the amount of deposition is controlled simply by controlling the amount of plating metal vaporized via control of the current through the filament on which the metal is melted and from which the metal is evaporated. This is generally inadequate for the fine-tuning required in many applications because it is difficult to stop the vaporization process at precisely the right moment to insure the desired frequency in the plated resonator. This is because of the tendency of the vaporization apparatus to overrun slightly when its control circuits are turned off.
There are several other known ways to control the plating process. One approach is to vary the deposition rate from an initial high rate to a lower rate as the resonator frequency approaches the target frequency Such an approach is taught in U.S. Pat. No. 3,670,693, by Rorick et al. Another approach is to interject a shutter system between the vaporization source and the object to be plated to abruptly terminate the plating process such as taught by U.S. Pat. No. 2,432,950, Turner et al. Yet another approach is to abruptly release the vacuum and inject an inert gas into the vaporization chamber as taught in U.S. Pat. No. 4,112,134 to Buynak et al). The shutter system seems to be the most easily controllable, but previous systems suffered from mechanical complexity within the vacuum apparatus. This is compounded when multiple elements are to be plated through multiple masks requiring multiple shutters. This requires the use of elaborate control systems.